This invention relates to the production of tubes from hollow workpieces, and in particular to methods and apparatus for facilitating the loading and unloading of such mills.
U.S. Pat. No. 4,090,386 relates to a method of producing zircaloy tubes where it is desired to have high rates of reduction in the wall thickness. The specification describes a McKay rocker wherein the mandrel which is employed in the reduction of the diameter of the tubing, is securely locked within a crosshead of the device along with the hollow or workpiece that will be reduced. During the forming or rolling operation, the crosshead advances the workpiece into a forming zone while the workpiece and mandrel are periodically turned by a clamp mounted on the crosshead.
U.S. Pat. No. 4,233,834 is similarly related to a method of producing zircaloy tubes and is specifically directed to tubing wherein the spiral formation of the wall thickness is controlled. As in the above described patent, the specification describes a McKay rocker mill wherein the mandrel and workpiece are securely clamped in a chuck which is located on a movable crosshead. As the crosshead moves the hollow or workpiece into the forming zone, the chuck periodically turns the workpiece.
U.S. Pat. No. 4,655,068 is directed to a method and apparatus of producing tubing generally and, in particular, is directed to a method of clamping a mandrel that is allowed to float in the workpiece. As in the above described U.S. patents, the specification describes a McKay rocker mill wherein a workpiece is moved into a forming or rolling zone by a movable crosshead while periodically being turned. However, in this arrangement, the mandrel is clamped in the crosshead with a force less than the mandrel's yield strength. Thus, tensile forces that might otherwise cause the mandrel to buckle while the workpiece is urged through the rolls are no longer applied to the mandrel.
Each of the above patents disclose methods that are, in varying degrees, acceptable for causing the workpiece along with the mandrel to travel through the tube-forming zone of the McKay rocker mill. That is, the methods for producing tubes disclosed therein are acceptable for the production of tubes in varying degrees of efficiency and economy. Over the course of practicing these methods, however, it has become apparent that a significant inefficiency and, therefore, an increase in cost is encountered not in the methods used to move the workpieces through the forming zone of the rolls, but in the operations required in reloading the mill.
Prior to understanding the problems in reloading the mill however, the general operation of the typical Mckay rocker mill must be understood. During a forming operation, the mandrel and workpiece are rotated and moved axially together, which common movement is made possible by the fact that the workpiece has been deformed tightly against the mandrel by the forming rolls. It will be appreciated, however, that at the beginning or start-up of a forming operation, the workpiece lies loosely on the mandrel. Hence, as the forming rolls begin to act against the workpiece, there occurs an undesirable tendency for the workpiece to be simply pulled axially relative to the mandrel, rather than being moved as a unit with the mandrel as is necessary for proper forming. Moreover, at the end of the forming operation, it is necessary to axially retract the mandrel relative to the formed workpiece to a loading position so that the mandrel can receive new workpieces to be formed. However, since the formed workpiece tightly grips the mandrel, there occurs an undesirable tendency for the formed workpiece to return with the mandrel.
In an attempt to control the workpiece in the above situations, it has been previously proposed, i.e., it is known, to provide a clamp which applies a radial clamping pressure to the workpiece in order to axially restrain the workpiece, However, that clamp did not perform acceptably because the clamping pressure required to hold the workpiece axially stationary during a loading operation was too great for the start-up procedure. That is, when a rolling operation commences, it is necessary to permit some axial movement of the workpiece, albeit under resistance, in order to prevent buckling of the workpiece as the workpiece is urged toward the rolls. If a weaker clamp is used instead, the clamping pressure is insufficient to immobilize the workpiece during the unloading operation.
Therefore, in lieu of relying on a clamp during unloading and start-up procedure, it has been customary for the operator to perform the unloading and start-up procedures in a deliberate and careful manner so as to exert proper control over the workpiece. However, this is slow and tedious and significantly retards the rate of production.
Furthermore, once a rolling operation is resumed and the workpiece has been sufficiently deformed to ensure no axial movement relative to the mandrel, the clamp discussed above is released and the workpiece and mandrel allowed to travel to the rolls unrestricted. It has been discovered, however, that the clearances thus allowed between the clamp jaws and the workpiece during forming are oftentimes detrimental to the quality of tube thus produced. The clearances allow for the workpieces to be imprecisely guided through the rolls thus causing various geometric inconsistancies and cosmetic deficiencies in the resulting tubes.